The invention relates to a system and method for determining degradation in an emission control system.
Engine and vehicle fuel efficiency can be improved by lean burn internal combustion engines. To reduce emissions, these lean burn engines are coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, additional three-way catalyst, sometimes referred to as a NOx trap or catalyst, is typically coupled downstream of the three-way catalytic converter, where the NOx trap is optimized to further reduce NOx. The NOx trap typically stores NOx when the engine operates lean and release NOx to be reduced when the engine operates rich or near stoichiometry.
One method to provide emission control in a lean burn engine uses a sensor downstream of the NOx trap. The sensor is capable of measuring an amount of NOx in exhaust gas exiting the NOx trap. Engine air-fuel ratio is changed from lean of stoichiometry to rich of stoichiometry when measured NOx reaches a predetermined threshold. Such a method is described in U.S. Pat. No. 5,942,199.
In particular, the sensor described in U.S. Pat. No. 5,942,199 is capable of measuring both NOx and oxygen partial pressure in exhaust gas. The sensor has a first chamber where exhaust gas first enters and where a measurement of oxygen partial pressure is generated from a first pumping current. Also, in the first chamber, oxygen partial pressure of the exhaust gas is controlled to a predetermined level. Next, the exhaust gas enters a second chamber where NOx is decomposed and measured by a second pumping current, knowing the predetermined level.
The inventors herein have recognized a disadvantage with the above approach. In particular, if the output of the NOx sensor inadvertently indicates a NOx concentration greater than the predetermined threshold when NOx concentration is actually less that the threshold, lean operation will be ended prematurely. In other words, lean operation will be ended when it is actually possible to continue lean operation.
An object of the invention claimed herein is to provide a system and method for determining degradation in an emission control system.
The above object is achieved, and disadvantages of prior approaches overcome, by a method for determining degradation in an emission control system coupled to an internal combustion engine comprising: providing first and second signals from a sensor coupled downstream of the engine, said first output signal and said second output signal respectively indicative of a first exhaust gas constituent flowing through the emission control system and a second exhaust gas constituent flowing through the emission control system; and determining degradation of said second output signal based on said first output signal.
Since a single sensor mutually supplies two signals, it is possible to determine when one signal-has degraded based on the other signal. For example, in one aspect of the invention, the sensor supplies indications of both oxygen partial pressure and nitrogen oxide. In this example, exhaust gases must first be affected to provide an indication of oxygen partial pressure. Then, the affected exhaust gases are used to provide an indication of nitrogen oxide. By observing the first output, oxygen partial pressure, it is possible to determine whether the second output, nitrogen oxide, is degraded. In other words, if the first output is degraded, the second output will also be degraded. This is because the functioning of the second output relies on operability of the first output.
An advantage of the above aspect of the present invention is improved control since it is possible to determine operability of sensors.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.